<!DOCTYPE HTML>
<html>
<body>
<canvas id="canvas" width="300" height="300"/>
<script>
var ctx = document.getElementById('canvas').getContext('2d');

var grad = ctx.createLinearGradient(150,0,150,300);
grad.addColorStop(0, 'white');
grad.addColorStop(1, 'black');

ctx.fillStyle = grad;
ctx.fillRect(0,0,300,300);
</script>
</body>
</html>
